The present invention relates to a multiple output switching power supply, and more particularly to a current type multiple output switching power supply suitable to supply a power to an OA or FA equipment which requires a high precision of output voltage.
A current type switching power supply is disclosed in Japanese Unexamined Patent Publication No. 55-120369 which discloses that a recovery current of a flywheel diode inserted in a secondary circuit of a transformer does not flow into a switching device in a primary circuit of the transformer and flow of an excessive current in an input circuit is suppressed even if the transformer is saturated by a surge voltage from the input circuit. However, it does not recognize a problem encoutered in the multiple output switching power supply.
Japanese Unexamined Patent Publication No. 55-144775 discloses a technique to operate a switching device to stabilize one of the outputs so that the multiple outputs are stabilized. In the disclosed technique, a choke coil inserted in a secondary circuit of a transformer has a plurality of magnetically coupled windings. In the disclosed technique, it is not possible to determine the output voltages by turn ratios of a plurality of secondary windings wound on a transformer but turn ratios of the windings wound
The choke coil should also be taken into consideration. As a result, the more the number of outputs is, the more difficult to design the power supply. Further, since one output is stabilized by the operation of the switching device, variations of other output voltages due to a variation of the one output voltage, that is, voltage variations due to so called cross-regulation increase. Causes of the cross-regulatin are leakage inductances and winding resistances of the transformer. In the disclosed technique, the leakage inductances and the winding resistances which cause the cross-regulation during an on period of the switching device affect to the transformer as well as leakage inductances and resistances of the windings of the choke coil and make if difficult to precisely control all of the output voltages.
In a technique disclosed in Japanese Unexamined Patent Publication No. 56-19119, an inductor is connected to an A.C. input. As a result , a ripple of a current flowing through the inductor is large, a current capacity of a rectifying/filtering circuit in a secondary circuit of a transformer is large and a maximum current flowing through a leakage inductor and a resistor of the transformer is large. As a result, outputs are affected by voltage drops thereacross and precision of the output voltages is lowered.